1. Wideband Gap Semiconductors and New Trends in Power Electronics
Department of Electronic & Electrical Engineering
Wideband Gap Semiconductors and New Trends in Power Electronics
Professor Peter R. Wilson
University of Bath 1

2. Some Fundamentals…get to with Silicon Devices and Power Electronics
Where did we get to with Silicon Devices and Power Electronics?
Some Fundamentals…get to with Silicon Devices and Power Electronics

3. Linear Power Supplies
The “Old” method used a linear power supply to rectify AC to DC
Standby power of a few W
Diode Power ~0.5W
Transformer Losses ~3.5W

4. The “New Way” – switching
By switching the power, we can take advantage of reduced inductor sizes to reduce the static power and “off” time losses
Diode Power ~0.5W
Transformer Losses ~0.2W

5. Where can we go next? We can use higher voltage power devices
GaN (600V) or SiC (1200V)
Why Not use IGBTs?
They are slower than MOSFETs
Using GaN or SiC allows much faster switching
Faster Switching Frequencies means transformer can be much smaller
Lower inductance L
We can reduce standby power to almost zero by using advanced controllers

6. What are the issues and sources of energy transfer and power loss
Managing power

7. Energy and Frequency
The Energy transferred per cycle can be defined using:
And the overall Energy by:
Therefore as the Frequency goes UP, the Inductance goes DOWN

8. Impact on Products
The impact on circuit design and product size can be dramatic
Reduction in Inductor and Capacitor Size
Simplification in Circuit Design

9. Losses There are two main Sources of Loss: Active Passive
Switching devices such as diodes & MOSFETs have switching losses that create heat
Passive
These fall into two further types, component and layout based losses.
Component losses may be obvious such as resistor loss, but also the resistive losses in inductors or capacitors
Layout based losses occur when current flows through other conductors, such as wires, PCB tracks, enclosures or other physical structures

10. Dynamic switching losses
Vload
Iload t
V,A V
Switch control ON
OFF
Von
Td(on)
Td(off)
tri
tfv
trv
tfi W
Pon
Tc(on)
Tc(off)
Vload*Iload Vd io
Switching times determine the switching Loss
On Resistance
determines the
On State Loss

11. Comparing new Power Devices with Silicon
Wide Band Gap Devices?

12. Standard HV MOSFET The Standard MOSFET has a BV of ~600V
The drift Region dominates Ron
Source
Gate Conductor
Field Oxide ``
p (body) ``
p (body)
Gate Oxide n+ n+ n+ n+
n- (drain drift)
~40mm n+
Drain

13. Breakdown and Mobility
In order to achieve sufficient breakdown we need to calculate the required mobility P N- N+ E
Emax L
To get Vbr ≥ 700V => N ~ 3.5E14 cm-3 => 40 µm depletion region

14. Device Sizing and Ron
In the Off state, In order to achieve the required breakdown voltage we need the depletion region to be an adequate size
For example, to get Vbr ≥ 700V:
In the ON state, the resistance will vary by:
Or more realistically:

15. The “Silicon Limit”
The relationship between the Breakdown Voltage and On Resistance defines the effective limits of the device operation
The “Silicon Limit”

16. Is this really the limit?
IGBT: these have been around since the 1980s and essentially consist of a MOSFET gate driver, and a PNP device
High power is possible, but is relatively slow
Lateral resurf (REduced SURface Field) devices
Originally developed at Philips, these devices use the P substrate to extend the depletion region and reduce Ron
Superjunction devices: rotating a resurf device so the same principle is implemented in a vertical device
Invented by Prof. Xingbi Chen.
Taken to market by Infineon
Despite these excellent developments, all have some limitations and only move some way from the Silicon Limit

17. Lateral Resurf devices
Lateral power devices can easily be integrated with an on-chip controller. Can sense temperature of power device directly.
Less costly packaging.
Vertical power device has high-voltage of back of die which in normally connected to the tab of the package  handling issues.
Controller
Vertical SJ MOS
Controller
Resurf
Power
MOSFET
A vertical power transistor needs a separate controller chip – either in a separate package, or as a hybrid (2-chip) package.
(images courtesy Martin Manley, Power Integrations Inc.)

18. GaN HEMT (High Electron Mobility Transistor)
Structure is quite different from conventional Si and SiC devices.
They are Hetero-junction devices – the current is carried in a 2-dimensional electron gas (2DEG), rather than an inversion layer.
2DEG is created by spontaneous polarization at AlGaN/GaN interface
Conduction is modulated by a gate electrode that overlays the 2DEG.
Silicon, sapphire or SiC substrate
Lattice matching “Buffer” layer
GaN
2DEG
(images courtesy Martin Manley, Power Integrations Inc.)

19. Material Properties How do the device materials compare?
SiC and GaN have
much higher critical field => higher breakdown voltage
Electron saturation velocity twice as high
SiC has excellent Thermal Conductivity
Property
Silicon (Si)
Silicon Carbide (SiC)
Gallium Nitride (GaN)
Band Gap (eV)
1.1
3.2
3.4
Critical Field (106V/cm)
0.3
3.0
3.5
Electron Mobility (cm2/Vs)
1450
900
2000
Electron Saturation velocity (106cm/s) 10 22 25
Thermal Conductivity (W/cm2K)
1.5 5
1.3

20. Extending the “Silicon Limit”
The characteristics of the Wide Band Gap devices (especially SiC and GaN) lead to an extending of the “Silicon Limit”

21. Advantages of WBG SiC has a similar structure to Si devices
Easier transition from Si to SiC for processing
SiC has MOSFETs and Diodes in production
Available NOW for Power Electronics Designers
Wafers more expensive, but in the same order of magnitude
SiC thermal Conductivity is excellent and is therefore easier to extract excess heat
SiC tolerates higher voltages (1200V is routine)
GaN only 600V
GaN limit is higher than SiC
GaN can operate at higher speeds than SiC
GaN reliability is a question for large scale deployment

22. Voltage/Frequency Context
SiC
1000V
Silicon
GaN
500V
100 1k 10k 100k 1M 10M 100M 1G 10G
Frequency

23. Challenges for designers
Compact Models (Challenge for researchers/Synopsys?)
We need new compact models for SiC and GaN
We need accurate parameters for components
Long Term Behaviour (challenge for vendors?)
Stress, Reliability and Thermal behaviour
Packaging
How do we model modules not just devices?
Gate Drivers
A specific and difficult issue with the devices possibly able to switch at 1MHz and beyond – inductive effects are critical
SiC devices need a larger voltage (~20V) than Si MOSFETs to switch properly and dedicated gate drivers are needed

24. What are the possibilities for Power Electronics?
ConCLusions

25. Conclusions
Wide Band Gap devices offer the possibility of lower On resistances for higher voltages
High Speeds of operation leading to smaller designs, smaller passives
Thermal tolerance is exceptional for SiC devices in particular
Issues remain with gate drivers and speed of operation – packaging and layout
Higher speeds, Higher Temperatures, More Reliable, Lighter, Smaller Circuits
an exciting time for Power Electronics!

26. Acknowledgments Martin Manley, Power Integrations Inc
Some figures
Interesting and Stimulating Discussions on Wide Ban Gap devices
IEEE Power Electronics Society
Funding a study into the reliability of SiC MOSFETs – results to be published in 2016